I've created a pretty detailed model of a college campus. It works pretty well for the most part, but at some points the frame rate goes completely to pot. I mean like down to 2 or 3 fps. I've got a couple ideas, but I'm not sure which to start with.

One place it really slows down is where I use a bunch of sprites for things like trees and statues. Also near that place, though, is a stairwell I cut into a block using CSG subtract. Well, I'd already hollowed the block so I could apply different textures to the various sides of it. So when I did the CSG subtract I ended up with an opening into the block.

Another place the level really slows down is close to another CSG subtract mistake, so I'm guessing that's the problem, not the sprites. After those two mistakes and reading several posts about the evil doings of CSG subtract I now steer clear of it, but do you think that could be causing the drop in frame rate?

What are some other things I should look for to speed up the frame rate?

Thanks,
Spaz